1. Field of the Invention
The present invention relates to the art of paper manufacturing and, more particularly, an innovative press section for the manufacture of paper.
2. Description of the Prior Art.
Conventional papermaking machines generally comprise a web forming section, a press section, and a drying section. Typically, within a web forming section, wet celluosic material, commonly referred to as stock, is collected in a headbox and then sprayed or deposited as a web of wet paper material onto wires. From its formation at the output of the headbox, the web is carried on wires (which are part of an apparatus known as a "fourdrinier") where the moisture evacuation process begins. Out of the web forming section, the web enters the press section where the web continues to be shaped and formed into the desired product. Major functions of the press section include moisture evacuation, primarily through a combination of compression (which occurs while the web is "rolled" through press nips), absorption (which occurs while the web is transported by moisture-absorbent felt), and suction (through one or more vacuum devices). Out of the press section, the paper web is transferred to the dryer section where it is dried, mostly through heating and evaporation.
Within press sections, pressure and vacuum devices and combinations thereof are commonly used to extract moisture from the web: devices such as press nips, shoe presses, suction rolls, perforated surface or blinddrilled rolls, and vacuum boxes. The paper web is carried through the press section, including a series of press nips, on the surfaces of one or more belts. Much of the evacuation occurs by compression through press nips which are formed between opposing cylindrical rolls. Commonly used are blinddrilled rolls, rolls having imperforate roll shells covered with a myriad of "blind" drilled holes and/or recesses. Blinddrilled rolls channel moisture away as the web is drawn through press nips and compressed. Vacuum devices, commonly known as vacuum boxes and suction rolls, as may be installed on the downstream side of nips to draw water away from the nip and prevent felt remoisturizing.
Many improvements in press section design are directed to the problem of water evacuation. This is because evacuation by heating and evaporation through drying sections consumes more time and energy than evacuation by mechanical means in press sections. Even marginal increases in moisture evacuation by mechanical means, through press section improvements, can result in corresponding improvements in manufacturing time, energy consumption and overall manufacturing cost.
One example of a press section improvement is the invention disclosed by Karvinen in U.S. Pat. No. 4,931,143, entitled Press Section with Separate Press Nips in a Paper Machine, issued Jun. 5, 1990. Karvinen discloses a series of three successive press nips in a closed draw configuration. Significant features of Karvinen include two press nips succeeded downstream by an extended nip wherethrough the web is layered between a non-water receiving fabric, situated underneath the web, and a water-receiving fabric which is dedicated to the extended nip and situated above the web. The extended nip employs a shoe press which is recognized for improved moisture evacuation qualities.
Another example is disclosed by Steiner, et al in U.S. Pat. No. 5,393,384, entitled Paper Machine for the Production of Tissue Paper, issued Feb. 28, 1995. Steiner includes a press section having at least one shoe press, a device for separating the paper web from the press felt after one press pass, press felts dedicated to each press nip, and a water impermeable belt. Steiner combines the shoe press with its moisture evacuation qualities and a nonabsorbent belt which prevents remoisturizing of the paper web, thereby improving press section moisture evacuation qualities over designs of prior art.
Although there has been substantial progress in optimizing moisture extraction through improvements in press section design, such efforts have introduced or increased other inefficiencies encountered in the paper manufacturing process. For example, the separate, moisture-absorbent belts which are dedicated to particular press nips, as taught by Karvinen and Steiner, may actually increase the number of components and moving parts required. Consequently, such designs may actually increase overall maintenance and operating costs, while some may increase the floorspace requirements over press sections of more common design. Furthermore, where press section componentry is spread over larger areas, the draw of the paper web may similarly increase, thereby introducing greater risk of web tearing and other deformities and increasing the overall cost of manufacture. Many of the problems of prior art designs, together with their associated manufacturing inefficiencies, are overcome by the present invention.